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Intertidal Interviews: Seaweed Stories on the North Yorkshire Coast

Our research began on the Scarborough coast, together with local experts and scientists we explored the coastline through, on and with the tides. Beginning on a boat with Sue Hull, then on foot with an intertidal seaweed walk with Jane Pottas, we investigated rock pools, collected specimens, explored each researcher's connection to the littoral space of the North Yorkshire Coastline and looked closely at the unassuming seaweed of the Scarborough shoreline. The following transcript combines interviews, email exchanges and informal conversations that we had in the bracing winds and drizzle of a UK seaside town.

Can you describe the diversity of seaweed on the Yorkshire coastline?

We have recorded over 90 species of seaweed on the Yorkshire coast – some of it is very common, other species are rare and some are only found at a limited number of sites. So, a lot of variety – from things that look like pink rocks, to the large forest forming kelps in deeper water.

There are about 650 species of seaweed in Britain – around 7% of the known seaweed flora, estimated to be between 12,000 and 15,000 species in the world. Britain is a seaweed hotspot because of its geographical position – it’s a Goldilocks zone where conditions are just right for many species. As well as being home to species with a cosmopolitan distribution, we find species with a northerly distribution at the southern edge of their range and others with a southerly distribution at the northern limit of their range. Some seaweeds are only found on the west coast – not on the Yorkshire coast, for instance! A single shore may have upward of 100 different seaweed species. Around 35 of the seaweeds found in Britain are non-native – they have arrived here as a result of human activity, accidental introductions carried here on the hulls of ships or with oysters imported for oyster farming. The first non-native seaweed in Britain was recognised in Falmouth in 1893 – nearly 125 years ago.

The ocean is arguably borderless, yet we still use binaries to classify that allude to an association or trespass of a specific place. Are there any native/non-native or maybe even invasive seaweeds to the Yorkshire coast?

The range of a species Is determined by the conditions found there. Some species have specific requirements for survival and have a narrow distribution range. If these species are moved to another location they may or may not survive and this applies to seaweeds, too. The 35 or so non-native seaweeds in Britain are the survivors – others may have been brought here but died out.Some of the non-native seaweeds on our coasts are benign – they arrive and settle; others are invasive and spread – and are still spreading.

The Yorkshire coast is currently doing really well in terms of resisting invasions – might be because the waters are a bit chilly and bracing, but we have very limited records of non-native seaweeds (mostly in ports) and none of them are invasive (phew). We have some invasive animals – but not seaweeds. So that’s all-good news for seaweed fans.

We also have the Flamborough Front on the east coast – this is the boundary between two water bodies in the North Sea where the cold deep waters of the Northern North Sea Cell meet the shallower, warmer waters of the Southern North Sea Cell. The front oscillates (depending on time of year and other factors) so it might be expected that there will be a certain amount of mixing of water and the possibility for southern species to cross and head north but so far it seems to have acted more like a barrier than a gateway. The Yorkshire coast is relatively pristine when it comes to non-native seaweeds but they may arrive here so we need to be on the look out for their arrival – we need to regularly survey the shores to know what is here already and recognise any newcomers as soon as they arrive so their spread (if they do) can be monitored. This is called horizon scanning.

A prime example of how one species spread with disastrous results is the case of Caulerpa taxifolia, a green seaweed with a wide distribution in tropical waters. It proved to be a successful plant in aquariums because it provided colour and produced oxygen in the water, and the fish didn’t eat it (unlike the other weeds) because if produces toxins to deter grazers.In 1984 a small patch was found growing in the Mediterranean Sea – as a result of aquarium tanks in Monaco being emptied into the sea, tanks which had been populated with C. taxifolia. The seaweed quickly spread and today there are thousands of hectares of it along the Mediterranean coast where it has excluded native seaweeds which in turn has driven away the fish that would have eaten them and this has impacted the inshore fishing industry. Once established it is almost impossible to eradicate aquatic species – outside its native range C. taxifolia is highly invasive – and one to watch out for in the UK. It could be brought to our shores on vessels passing through the Gibralter Straits and once here could spread like wildfire. With warming seas northern species retreat and southern ones can extend their range north, thus changing the biodiversity on our shores.

When standing in the littoral zone in Scarborough, and similarly on much of the UK coastline, seaweed appears to be prolific, yet you spoke of how challenging it can be to grow seaweed in aquariums. How specific are the needs of seaweed species to grow?

It’s an interesting point. The subject of my PhD, the brown seaweed Fucus spiralis, is a common species in the North Atlantic, which grows in the upper reaches of the shore. It can tolerate a wide range of temperature and salinity, and it produces chemicals to protect itself from desiccation and grazers. On warm, windy days it can lose up to 95% of its water content, yet when the tide rises, within a very short time of being wet again, it can start metabolising once more. Part of my research was a simple experiment: I placed some specimens of F. spiralis in a seawater tank and kept them immersed 24/7. After a few days they began to disintegrate, suggesting that this species has to experience a period of exposure and cannot withstand total and prolonged immersion. Seaweeds which grow between high and low water are distributed across the littoral zone depending on how long they can tolerate total immersion and exposure. Similarly, seaweed distribution is influenced by a range of biotic and abiotic factors and because different species have different requirements for survival, they do not all have the same distribution patterns which explains why the range of species found on Yorkshire shores is not the same as ones found in other parts of Britain.

Have you noticed any stress impacts on seaweed or shifts in species that you are seeing more of?

We have noticed that certain species have suffered due to the recent heat waves – the pretty pink Coralweed that is common in rockpools bleaches to white and dies back, and other red seaweeds also lose their colour and bleach when exposed to long periods of strong sunlight. The red seaweeds are the most sensitive as they are not very well adapted to strong sunlight and high temperatures, as are the kelps that are usually not out of water for long. We are worried that if the warming persists it will impact these quite severely. The kelps that have suffered bleaching when exposed to strong sunlight at the moment seem to be recovering. We are watching various sites where kelp has declined to see if it comes back – this could be really worrying, but we need to be able to monitor if things recover.

And what do you speculate or imagine the future of seaweed to be on the Yorkshire coastline?

This all depends on how warm it gets and what happens with our weather – a very British thing to say. Yorkshire could be a seaweed stronghold as we have quite cool sea temperatures compared to some areas of the UK – so we may continue to see a lot of our species persist. But we may lose the more boreal ones – i.e. those that extend up to the Arctic, which will not like the warmer temperatures. If the seaweeds change, then the whole ecosystem will change – grazers, predators that feed on grazers etc. and there could be knock-on effects on commercial activities too.

So, there is a bewildering diversity of form, function and diversity on our coasts in Yorkshire. Some are really beautiful and just strands of single cells, others are monsters – the 3m long kelps in deeper water that act as nursery grounds for fish. They all act like a marine version of a woodland/grassland ecosystem that supports a really diverse suite of organisms that are all interlinked. From the smallest snail to the largest seal, seaweed provides a habitat, food, shelter and place to live and many organisms depend on it for survival.

Conversations, emails, voice memos and zoom calls were conducted between September and December 2023. Facilitated and edited by Kaia Goodenough.Dr Sue Hull is an intertidal marine ecologist trying to understand the dynamics of the communities inhabiting rocky coasts, especially those on man-made structures and how we can use ecological engineering to enhance biodiversity.Sue led the University of Hull hub for the national 'Capturing the Coast' team training citizen science volunteers to survey rocky shores alongside marine biologists to answer key science questions. This successful project has led to a better understanding of our marine environment and engaged people from all walks of life in surveying and understanding our coastal areas.Dr Jane Pottas is an intertidal ecologist who focuses on intertidal benthic seaweed communities, seaweed identification and distribution. In her role as a Research Assistant in the Cryptogamic Herbarium at the Natural History Museum in London, she was privileged to work with their wonderful collection of seaweed specimens on a number of projects, one with a particular emphasis on non-native seaweed species in Britain. She was a Field Officer with the citizen science project, Capturing our Coast. She is also a Past President of the Yorkshire Naturalists' Union.